LED display apparatuses including LEDs are widely used to display, for example, advertisements indoors and outdoors owing to technical development associated with LEDs and reduction in cost of LEDs. In particular, the LED display apparatuses have been mainly used to display moving images such as natural pictures and animations. In recent years, the LED display apparatuses have reduced pixel pitches to keep the display quality at shorter visual distances, and thus are also available for indoor use, for example, for use in meeting rooms and for monitoring.
The LED display apparatuses for use in monitoring often display images similar to still images on personal computers. The luminances of the individual LEDs decrease with increasing lighting period. Thus, depending on contents of images, the lighting periods of the individual LEDs are varied and the luminance decrease rates of the individual LEDs are varied accordingly. Consequently, the pixel-to-pixel variations in luminance and color occur over the lighting period.
The following methods have been proposed to reduce such variations in luminance and color. According to one method (see, for example, Japanese Patent Application Laid-Open No. 11-015437 (1999)), the luminance of the LED display is detected, and then the luminance is corrected. According to another method (see, for example, Japanese Patent Application Laid-Open No. 2006-330158), the display periods of the individual LEDs are accumulated, and then a luminance correction factor is corrected in accordance with the accumulated period obtained by the calculation, so that the luminance is corrected.
Variations in luminance and color caused by differences in lighting periods of LEDs can be corrected by measuring luminance decrease rate of the LEDs in accordance with the accumulated period in life tests and by correcting the luminance using the luminance decrease rate. However, different LEDs inevitably have different characteristics that are difficult to predict, such as characteristics that vary from production lot to production lot. It has been therefore difficult to accurately correct variations in luminance merely in accordance with the accumulated period.
Meanwhile, the luminance can be accurately corrected by detecting the luminance from the LED display that displays a desired image. However, this technique necessitates the displaying of an image for measuring luminance. Thus, it has been necessary to halt the displaying (operation) to be performed by a 24-hour operation display system (such as a display system for use in monitoring as mentioned above) in order to correct, for example, variations in luminance, or it has been necessary to give up the correction of variations in luminance and the like in order to keep the displaying of the desired image.